Usually, references to the interior are only created by the `Deref` and
`DerefMut` impl of the guards `Ref` and `RefMut`. Note that `RefCell`
already has to cope with leaks of such guards which, when it occurs,
effectively makes it impossible to ever acquire a mutable guard or any
guard for `Ref` and `RefMut` respectively. It is already safe to use
this to create a reference to the inner of the ref cell that lives as
long as the reference to the `RefCell` itself, e.g.

```rust
fn leak(r: &RefCell<usize>) -> Option<&usize> {
    let guard = r.try_borrow().ok()?;
    let leaked = Box::leak(Box::new(guard));
    Some(&*leaked)
}
```

The newly added methods allow the same reference conversion without an
indirection over a leaked allocation and composing with both borrow and
try_borrow without additional method combinations.